Peter's Blog

A quick reminder on how to use Swig under Windows XP to generate Python modules with Visual Studio 6.

• Download Windows version of Swig
• Unzip it somewhere, I put it in c:/Python24/Swig
• Create the following environment variables:

  PYTHONINCLUDE = c:\python24\include
  PYTHONLIB = c:\Python24\Libs
  

• Create a project in Visual Studio for a plain empty win32 dll.
• In all build settings, make sure the following settings are added:

  Additional include directories = c:\python24\include
  Additional library path = c:\python24\libs
  

• Create swig definition file. Here is mine, it's for a CAN bus control module:

     1  %module pyCan
     2  %include exception.i
     3  %{
     4  #include <windows.h>
     5  #include <stdlib.h>
     6  #include <stdio.h>
     7  #include <string.h>
     8  #include "pyCan.h"
     9  %}
    10  
    11  /*
    12   * When a c++ exception occurs, this code will turn it into
    13   * a python exception. Cool.
    14   */
    15  %exception {
    16      try {
    17          $action
    18      } catch( char *strError) {
    19          PyErr_SetString(PyExc_RuntimeError, strError);
    20          return NULL;
    21      }
    22  }
    23  
    24  /*
    25   * The function GetRxPacket returns a new object that python
    26   * memory management must delete when required.
    27   */
    28  %newobject GetRxPacket;
    29  
    30  /*
    31   * Class that will become a python class. The python class will have
    32   * the same methods available.
    33   */
    34  class pyCan {
    35  public:
    36      pyCan( void);
    37      ~pyCan( void);
    38      void Initialise( void);
    39      void Poll( void);
    40      pyCanPacket *GetRxPacket( void);
    41      void TxPacket( int nID, PyObject *);
    42  };
  

• Add the Swig file to the project, right click and select 'settings'. Set the custom build commands something like the following:

  c:\python24\swig\swig -c++ -module pyCan -python c:\projects\694\src\pyCan\pyCan\pyCan.swig
  
  Where
  
      -c++ = generate c++
      -module pyCan = output module is called 'pyCan
      -python = generate python module
      c:\projects\694\src\pyCan\pyCan\pyCan.swig = swig file name
  

• The above will generate a c++ file called 'pyCan_wrap.cxx'. This should be put into the 'Outputs' box in the custom build step.
• Run the build on the swig file to create the wrapper file pyCan_wrap.cxx and add that file to the project.
• Set the link target to something like

  _pyCan.dll
  

  I.e. the name of the module with an underscore in front.
• Create a 'real' c++ header and implementation for your classes. They can be totally straight c++, e.g.:

  class pyCan {
  public:
      pyCan( void);
      ~pyCan( void);
      void Initialise( void);
      void Poll( void);
      pyCanPacket *GetRxPacket( void);
      void TxPacket( int nID, PyObject *);
  };
  

  In the above example I am passing a PyObject * to the function TxPacket. This is because I want to be able to pass python strings that may contain null characters and I want access to both the pointer to the data and the data length, which the raw PyObject gives me. If I only wanted a char * I would simply put that in the argument.
• Build it
• Swig will create a file called 'pyCan.py' which is the python wrapper module that loads the dll. It is the file that your python must import. You might want to add a post-build step to copy this somewhere more useful. You may also put the link target file in the same directory or you will get errors when you import the module.
• Write some python:

  import pyCan
  
  o = pyCan.pyCan()
  o.Initialise()
  while 1:
      o.Poll()
      r = o.GetRxPacket()
      if r:
          print r
  

Swig is quite easy to get going. I think it has real power but there is a lot of documentation to go through to fully understand what it can do and the documentation and semantics are not all that clear. The PyObject trick above was a guess of mine that happened to work.